This invention relates to optical systems for digital cameras and, in particular, to optical systems which permit through-the-lens (TTL) viewing of a scene to be digitally recorded (photographed).
There exists a need in the art for digital cameras having some and preferably all of the following properties:
(1) The camera is a TTL camera so that parallax problems are avoided, thus making the scene viewed by the user and the scene recorded by the camera as close to identical as possible.
(2) During picture composition (picture framing), the camera is completely optical as opposed to being a combination of an optical system and an electronic display, e.g., an LCD display. In this way, the amount of electrical current used by the camera is minimized and thus battery life is maximized. As known in the art, achieving long battery life is one of the key problems in designing a successful digital camera.
(3) The camera has a recall mode (also known as a review mode) so that the user can view previously recorded images in real time.
(4) The recall mode uses an electronic display (display unit or microdisplay) to reduce camera size and conserve battery power, but still creates a large recalled image for viewing by the user, i.e., an image which subtends a large angle at the user""s eye. A large recalled image is important since it allows the user to perform recall without the need for, for example, reading glasses.
(5) The camera has a single eyepiece which is used in both the TTL and recall modes. In this way, the user can perform recall without taking his or her eye away from the position used to view the scene to be photographed.
(6) The camera is a zoom camera with a focal length range of preferably at least 2:1 and most preferably at least 3:1.
(7) The camera is ergonomic and is of relatively small size both during use and storage.
(8) Although being small in overall size, the camera has a large zoom space thus allowing the use of a negative-positive (NP) relaxed zoom objective lens.
(9) The camera has a mechanical construction which is compatible with production of high quality images. In the past, many digital cameras have employed the classical layout of film cameras, namely, a layout in which the optical axis of the camera is parallel to the camera""s shortest dimension. To provide for compact storage, such digital cameras have had collapsible optics, i.e., a lens barrel which was extended from the camera during picture taking and folded back into the camera during storage. For zoom lenses, it is difficult to maintain good optical performance with collapsible optics since the collapsing action compromises concentricity and accurate positioning of the optical elements making up the lens.
(10) To reduce manufacturing costs, the camera""s optical system employs relatively few optical elements and is designed to have a low sensitivity to manufacturing variations (tolerances).
In view of the foregoing, it is an object of the invention to provide digital cameras which have some and preferably all of the above features. It is also an object of the invention to achieve the foregoing object through optical systems that are suitable for integration with a digital camera""s electronic components in an overall small package.
To achieve the foregoing and other objects, the invention in accordance with a first aspect provides an optical system which can be switched between a first optical path (1) and a second optical path (2), said system comprising:
(A) an objective lens unit (10) having a positive optical power;
(B) an erector lens unit (12) having a positive optical power;
(C) an eye lens unit (13) having a positive optical power;
(D) a display unit (16); and
(E) optical means (17,18) for switching the optical system between said the first (1) and second (2) optical paths;
xe2x80x83wherein:
(i) the first optical path (1) comprises, in order, the objective lens unit (10), the erector lens unit (12), and the eye lens unit (13); and
(ii) the second optical path comprises, in order, the display unit (16) and the eye lens unit (13).
In accordance with a second aspect, the invention provides an optical system which can be switched between a first optical path (1), a second optical path (2), and a third optical path (3), said system comprising:
(A) an objective lens unit (10) having a positive optical power;
(B) an erector lens unit (12) having a positive optical power;
(C) an eye lens unit (13) having a positive optical power;
(D) a sensor unit (15);
(E) a display unit (16); and
(F) optical means (17,18) for switching the optical system between said first (1), second (2), and third (3) optical paths;
xe2x80x83wherein:
(i) the first optical path (1) comprises, in order, the objective lens unit (10), the erector lens unit (12), and the eye lens unit (13);
(ii) the second optical path (2) comprises, in order, the display unit (16) and the eye lens unit (13); and
(iii) the third optical path (3) comprises, in order, the objective lens unit (10) and the sensor unit (15).
In certain preferred embodiments of the foregoing two aspects of the invention, the second optical path (2) comprises the erector lens unit (12) between the display unit (16) and the eye lens unit (13). The use of this unit in the second optical path, however, is not required since the eye lens unit (13) and the display unit (16) can be designed to operate without an erector lens unit, e.g., the display unit can display a reverted and inverted image and the magnification of the eye lens unit and/or the size of the display unit can be chosen so that the image of the display unit produced by the eye lens unit subtends a sufficiently large angle at the user""s eye for ready viewing of recalled pictures.
In other preferred embodiments of the foregoing two aspects of the invention, the system further comprises an image size adjusting lens unit (11) located in the first optical path (1) between the objective lens unit (10) and the erector lens unit (12).
In still further preferred embodiments of the foregoing two aspects of the invention, the optical system has an optical axis (1,2) which lies in a single plane for the first optical path (1) and the second optical path (2).
In certain preferred embodiments of the second aspect of the invention, a sensor lens unit (14) is located in the third optical path (3) between the objection lens unit (10) and the sensor unit (15) and serves to reduce the sensitivity of the system to manufacturing variations.
In accordance with a third aspect, the invention provides an optical system comprising:
(A) a zoom objective lens unit (10) which has a long conjugate and a short conjugate and, in order from said long conjugate to said short conjugate, comprises:
(i) a first lens unit (U1) having a focal length fi, said first lens unit comprising two lens elements, each of which has two aspherical surfaces;
(ii) a second lens unit (U2) that moves to effect a change in the optical power of the zoom objective lens unit, said second lens unit having a focal length f2 and comprising two lens elements, each of which has two aspherical surfaces; and
(iii) a sensor lens unit (14); and
(B) a sensor unit (15) adjacent to the sensor lens unit (14). In accordance with a fourth aspect, the invention provides a zoom lens (10) which has a long conjugate and a short conjugate and, in order from said long conjugate to said short conjugate, comprises:
(a) a first lens unit (U1) having a focal length f1, said first lens unit comprising two lens elements, each of which has two aspherical surfaces; and
(b) a second lens unit (U2) that moves to effect a change in the optical power of the zoom lens, said second lens unit having a focal length f2 and comprising two lens elements, each of which has two aspherical surfaces;
xe2x80x83wherein:
f1 less than 0,
f2 greater than 0, and
|f1|/f2 less than 1.
In certain preferred embodiments of the third and fourth aspects of the invention, the zoom objective lens unit or zoom lens has:
(a) a wide angle focal length fMIN; and
(b) an exit pupil distance DEP in short conjugate space;
where |DEP|/fMINxe2x89xa72.0 and preferably |DEP|/fMINxe2x89xa72.5. As used herein, xe2x80x9cexit pupil distance in short conjugate spacexe2x80x9d is the distance between the exit pupil and the image at fMIN.
In other preferred embodiments of the third and fourth aspects of the invention, the combination of the first and second lens units of the zoom objective lens unit or zoom lens has a back focal length DBF in short conjugate space such that DBF/fMINxe2x89xa71.0 and preferably DBF/fMINxe2x89xa71.5.
In still further preferred embodiments of the third and fourth aspects of the invention, |f1|≈|f2| e.g., |(|f2|xe2x88x92|f1|)|/|f2| is less than or equal to 0.2 and preferably is less than or equal to 0.1.
In accordance with a fifth aspect, the invention provides a camera which has an optical axis (1) and comprises:
(A) an objective lens unit (10) having a positive optical power;
(B) an erector lens unit (12) having a positive optical power; and
(C) an eye lens unit (13) having a positive optical power;
xe2x80x83wherein:
(i) the optical axis (1) lies in a plane and has first and second folds (17,20) which define a first optical axis portion (22), a second optical axis portion (23), and a third optical axis portion (24);
(ii) the first (22) and third (24) optical axis portions are parallel but not collinear;
(iii) the second optical axis portion (23) is perpendicular to the first (22) and the third (24) optical axis portions, with the first optical axis portion (22) extended in one direction from the second optical axis portion (23) and the third optical axis portion (24) extended in the opposite direction from the second optical axis portion (23); and
(iv) the optical axis of the objective lens unit (10) is collinear with the first optical axis portion (22), the optical axis of the erector lens unit (12) is collinear with the second optical axis portion (23), and the optical axis of the eye lens unit (13) is collinear with the third optical axis portion (24).
In certain preferred embodiments of the fifth aspect of the invention, the length of the first optical axis portion (22) between the object end of the objective lens unit and the first fold (17) is L1, the length of the second optical axis portion (23) between the two folds (17,20) is L2, the length of the third optical axis portion (24) between the second fold (20) and the image end of the eye lens unit (13) is L3, and
1.25(L1+L3)xe2x89xa7L2xe2x89xa70.75(L1+L3).
More preferably, L1, L2, and L3 satisfy the relationship:
1.15(L1+L3)xe2x89xa7L2xe2x89xa70.85(L1+L3).
In the preferred embodiments of the invention, L2 is less than or equal to 10 centimeters and preferably is less than or equal to 8 centimeters.
The foregoing aspects of the invention can be used alone or, preferably, in combination. When used in combination, digital cameras having integrated electronics and optics and having all of the desired features described above can be readily manufactured and used by professional and amateur photographers to frame, record, and recall high quality digital images.
Examples of zoom lens having a negative-positive (NP) configuration and employing aspherical surfaces include:
(1) U.S. Pat. No. 4,469,412 to Tajima et al. which illustrates the use of a single aspherical surface. While this approach can help to improve the optical performance of a lens at low cost, the use of one aspherical surface does not provide a very significant improvement in a cost/performance analysis.
(2) U.S. Pat. No. 4,560,253 to Ogino illustrates the use of a thin plastic substrate on a glass element to form an aspherical surface. In this design, the benefit of a glass lens element is maintained while the cost of adding the aspherical surface is modest. Only one aspherical surface, however, does not offer a significant cost/performance improvement.
(3) U.S. Pat. No. 5,054,897 to Ozawa is an example of using several aspherical surfaces, but in this case there is only one aspherical surface for any particular lens element. By restricting the application of aspherical surfaces to one per lens element, the ability to reduce sensitivity to manufacturing deviations by appropriate aberration balancing is minimal.
(4) In Japanese Laid Open Application 7-225338, an NP design is disclosed having two lens elements in the positive second lens unit that are aspherical on both surfaces. The first unit, however, includes only spherical elements, which makes the lens sensitive to manufacturing variations. In addition, the optical power distribution of this design is not adequate to provide for an extended exit pupil position required by some sensor units, nor would this type of design allow for the insertion of mirrors in the image space.
An example of the use of a relay lens unit to invert an image produced by an objective lens is shown in U.S. Pat. No. 5,523,885 to Aoki.